This invention relates generally to position/orientation tracking and, in particular, to methods and apparatus for accurately tracking position, orientation and movement within a volume in the presence of electromagnetic distortion and/or noise.
Position and orientation tracking systems (xe2x80x9ctrackersxe2x80x9d) are well known in the art. For example, U.S. Pat. Nos. 4,287,809 and 4,394,831 to Egli et al.; U.S. Pat. No. 4,737,794 to Jones; U.S. Pat. No. 4,314,251 to Raab; and U.S. Pat. No. 5,453,686 to Anderson, are directed to AC electromagnetic trackers. U.S. Pat. No. 5,645,077 to Foxlin discloses an inertial system, and combination systems, consisting or two different trackers, such as optical and magnetic, are described in U.S. Pat. No. 5,831,260 to Hansen and U.S. Pat. No. 6,288,785 B1 to Frantz et al. Other pertinent references include U.S. Pat. No. 5,752,513 to Acker et al. and U.S. Pat. No. 5,640,170 to Anderson.
AC electromagnetic trackers have definite advantages over other types of systems. For one, AC trackers provide the highest solution/update rate with the greatest accuracy, not affected by obstructed field of view, in contrast to optical solutions. AC trackers do not require reference sensor/unit and drift stable apparatus of the type required by inertial units, and they are not affected by the Earth""s magnetic field and ferrous materials, in contrast to DC magnetic systems.
The main disadvantage of AC trackers is that they are quite susceptible to distortion due to eddy currents in conductive materials in or near the motion box. To overcome this phenomenon, magnetic trackers often require costly and time-consuming calibration/mapping procedures to function correctly in the distorted environment. With mapping, the magnetic field profile is measured at multiple points associated with the volume of interest (motion box) prior to the actual tracking, as discussed in commonly assigned U.S. Pat. No. 6,377,041 to Jones et al., and some of the references cited therein. While mapping may be done quickly and accurately, any changes in the motion box will require repeating of the mapping procedure.
Another approach, described in U.S. Pat. No. 6,147,480 to Osadchy et al., allows the AC tracker to trace moving metal (distortion) by measuring the signal without distortion (acquiring baseline signals). This signal is then compared with a signal in the presence of distorting object(s) by measuring the phase error of the received signal. While such a system does work, it is not always practical to acquire baseline signal without distortion; in many cases, in an aircraft cockpit, for example, the distortion is always present.
The approach described in U.S. Pat. No. 6,172,499 B1 to Westley introduces at least two frequencies per source channel, and uses the difference in responses to compensate for the eddy current distortion. This approach requires a guess about the eddy currents loop geometry, and the efficiency of the distortion compensation and operational frequencies depends on assumptions regarding the distorted environment, including the physical characteristics of the distorting materials where the system will be working. In addition, this approach requires a comparatively wide-band receiver (sensor and ADC processing), thus reducing noise stability.
The methods and apparatus for distortion compensated AC tracking described in our commonly assigned U.S. Pat. Nos. 6,400,139 and 6,369,564, both to Khalfin et al., take advantage of wired xe2x80x9cwitnessxe2x80x9d sensors to obtain real-time information concerning the distortion (this is done by analyzing field profile that is superposition of the source field and distortion fields at the locations of xe2x80x9cwitnessxe2x80x9d sensors given sufficient xe2x80x9cwitnessxe2x80x9d sensors data). In addition, the ""564 patent describes the signal processing from a resonantly tuned wireless passive sensor, 90xc2x0 phase shifted with respect to the source to enable separation of the distortion signal.
Despite these advances, the need remains for apparatus and methods of compensation for spurious, eddy-current-induced fields in AC electromagnetic tracking systems. Such a solution could take advantage of the fact that the electromagnetic coupling which creates these eddy currents is strongly dependent on the frequency of the transmitted AC magnetic field. In addition, eddy currents are phase shifted with respect to the magnetic tracker source drive current that generates the magnetic field.
According to the system and method described herein, a bounding box or volume of interest is flooded with a modulated AC electromagnetic signal from a source. Different types of modulated signals may be used, including single-tone AM and FM. One or more sensors disposed on an object or body within the volume are then used to detect the signal, and a digital and/or analog spectral and phase analysis is performed on the received signal in hardware or software. The processing distinguishes between the direct source to sensor response and the response due to eddy currents. After removing the response due to the distorters, the electromagnetic position/orientation problem can be treated through a conventional xe2x80x9cfree-spacexe2x80x9d solution.
The disclosed system and methods do not require witness sensors, though the approach may be used in a combination with them. The invention operates in a narrow frequency band to ensure noise stability, and preferably uses high operating frequencies (e.g., of about 20 kHz-50 kHz) to ensure high signal quality and increased operation range.
A rapid solution update rate may be used to achieve real time (per frame) distortion compensation without any prior knowledge about physical properties of the distorters. At the same time, the system preserves all known advantages of AC trackers, but without the need for a calibration/mapping procedure, which has proven to be the main obstacle to more widespread applications of AC electromagnetic tracking technology.
The invention finds applicability in a wide variety of environments, including head tracking systems and helmet-mounted displays for fighter aircraft; head trackers for armored vehicles; medical-guided surgery and biopsy; remote sensing, among other potential uses.